We propose to test the hypothesis that pulmonary gas transport during mechanical ventilation of the lungs with high frequency oscillation (HFO) is a function of two major components: l) longitudinal transport along one pathway, 2) interpathway gas transport, which can occur at every bifurcation in the airways and through collateral channels. We propose to estimate during HFO the longitudinal conductance (Glongi) along pathways and the interpathway conductance (Ginter). We will also determine during HFO the vertical distribution of pulmonary blood flow and the ratio of regional conductance to regional perfusion (Qr). The effect of HFO on phases I and II and on the slope of phase III of the single breath oxygen test will be examined to assess gas transport in the airways and to estimate the magnitude of intraregional gas transport during HFO. The determinants of the conductance may include the mechanical properties of airways. Hence, species with distinctly different mechanical behavior of airways (pig, dog, human) will be studied. Conductance also depends on stroke volume and oscillation frequency. We will therefore determine the components of conductance at various combinations of oscillation frequencies and stroke volumes. In order to study the effect of the mechanical properties of lung parenchyma, conductance will be studied in dogs with induced lung disease. Knowledge of the relative magnitudes of the components of conductance and an understanding of the determinants of the components will contribute significantly to the understanding of pulmonary gas exchange during HFO in healthy and diseased lungs. This is of clinical importance because HFO may provide a useful alternative method of mechanical ventilation for patients requiring mechanical ventilation. The low airway pressure associated with HFO may reduce the incidence of barotrauma and the relatively motionless chest may make HFO an ideal method of ventilation during thoracic, cardiac, and abdominal surgery.